This invention relates to communication systems and, more particularly, to digital data communication systems. One application of the invention illustrated and described herein is an aircraft avionics system; however, the invention may be used in other applications and environments.
Commercial aircraft digital avionics systems typically are made up of multiple system elements such as inertial navigation system elements, autopilot system elements or electronic engine control system elements. Digital data is transferred over a digital data bus from each system element that acts as a source of data to one or more receiver system elements or data sinks that require the data over a digital data bus. Each system element is composed of either a data source or a receiver, as the case may be, and an interface acting between it and the data bus. Typically, one data source serves multiple receivers and, since the receivers do not also act as data sources, the data transfer via the data bus is uni-directional from a data source to one or more receivers.
Since the system elements typically operate at different data transfer rates, different interfaces are required, depending upon the data rate at which the associated data source or receiver operates. For example, an interface that is designed for use with a data source which operates at a low data rate generally is not suited for use with a data source that operates at a higher data rate. The source interface, or transmitter, controls the rise and fall times of the digital signals applied to the bus in order to minimize the electromagnetic interference radiated from the bus at that data rate. At the low data rate, the interface provides a relatively long rise time for the data transmitted on the bus. However, when operating at the higher data rate, the long rise time can mask succeeding bits and cause erroneous data to be transmitted. Likewise, an interface that is designed for use with a receiver which operates at a low data rate is generally not suitable for use with a receiver that operates at a higher data rate. For example, one of the functions of a receiver interface is to indicate to the receiver when the data transmission begins and ends. The signal chosen to indicate the presence of transmitted data must so indicate for a period of at least beyond the end of the last bit period received in order to avoid missing a data bit. When operating at a low data rate, the interface extends this signal for a predetermined period of time. However, when reception is at a higher data rate, the signal extension tends to overlap the start of the first bit period of a subsequent transmission.
The next generation commercial airplane will be required to accommodate new sensors and provide compatible interfacing systems, controls and instruments and may use approximately 100 data transfer busses per ship set. Until this invention, many different digital data transmission standards, using different word labels, formats and electrical characteristics have been required. In addition, previous digital avionics systems did not require data transmission freuqencies of 100 Kilobits per second (KBPS). Due to the new generation of digital avionics subsystems, a 100 KBPS data transmission frequency will be needed to support new sensors, controls and instruments. Examples of subsystems which require 100 KBPS data rates are new compass displays and electronic attitude direction indicators (EADI) which utilize CRT displays.
Therefore, it is an object of this invention to provide a digital data communication system capable of fulfilling the needs of future commercial aircraft avionics systems, in which information must be transferred via a digital data bus from a data source to one or more receivers in an economical and efficient manner.
Another object of this invention is to provide a digital data communication system having a source interface responsive to digital source data for encoding and transmitting it in digital signals on a digital data bus, and a receiver interface responsive to the encoded digital signals for decoding them and providing binary data representative of the digital source data.
A further related object of this invention is to provide a source interface adaptable for controlling the rise and fall times of the encoded digital signals on the data bus for a plurality of data rates.
Still another related object of this invention is to provide a receiver interface adaptable for receiving the encoded signals from the bus for a plurality of data rates and decoding the signals into digital data.